Question

    #include 'iostream'      class A {         private:         int a;         public :          A(): a(-1) {}         int getA() {             return a;         }      };      class A;      class B : public A {         private:         int b;         public:          B() : b(-1) {}          int getB() {             return b;         }      };      int main() {         std::auto_ptr<A> a = new A();          std::auto_ptr<B> b = dynamic_cast<std::auto_ptr<B> > (a);          return 0;      } 

ERROR: cannot dynamic_cast `(&a)->std::auto_ptr<_Tp>::get() const

Answer 1

Well, std::auto_ptr<B> is not derived from std::auto_ptr<A>. But B is derived from A. The auto_ptr does not know about that (it’s not that clever). Looks like you want to use a shared ownership pointer. boost::shared_ptr is ideal, it also provides a dynamic_pointer_cast:

boost::shared_ptr<A> a = new A(); boost::shared_ptr<B> b = dynamic_pointer_cast<B> (a); 

For auto_ptr, such a thing can’t really work. Because ownership will move to b. But if the cast fails, b can’t get ownership. It’s not clear what to do then to me. You would probably have to say if the cast fails, a will keep having the ownership – which sounds like it will cause serious trouble. Best start using shared_ptr. Both a and b then would point to the same object – but B as a shared_ptr<B> and a as a shared_ptr<A>